The present invention relates to a method of manufacturing cement by firing at high temperature a mixture of limestone and clay, or a mixture of silica, lime, alumina and iron oxides, adding gypsum to the product resulting from this firing, and crushing of the final mixture thus obtained.
As is well known, the manufacture of cement by the usual method mentioned above necessitates use of a large amount of heat energy. This energy is generally supplied by the combustion of a suitable fuel, such as coal, liquid hydrocarbons or natural gas, and firing the mixture of the starting ingredients at high temperature. In fact, the amount of heat energy required for manufacturing 1 kg of cement clinker is on the order of 700 to 1500 kilocalories.
It is also well known hat the amount of refuse and residues, such as household refuse and industrial waste, which needs to be eliminated, increases constantly in the industrialized countries. The elimination of refuse and residues poses problems which are becoming more and more difficult to solve.
More precisely, refuse is generally either simply stored in rubbish heaps, or incinerated and pyrolysed. These two procedures present serious disadvantages which are well known. In both cases there is a very high risk of pollution. In particular, in the case of incineration and pyrolysis, certain substances which are very often present in refuse are capable of releasing toxic gases into the atmosphere. Severe limitations thus result in practicing this treatment of rubbish and waste.
The progressive accumulation of considerable volumes of untreated waste or of residues from incineration incapable of any subsequent use, likewise poses serious problems.
The composition of refuse varies widely and in a more or less random manner as a function of the place, the conditions of its collection, and, likewise, according to the season. However, the composition lies generally within certain limits, which, in the industrial countries, are approximately as follows (expressed in percent by weight):
______________________________________ Paper and cardboard 45 to 53 Rags and other textile matter 10 to 13 Metals 7 to 9 Glass 8 to 12 Stoney and sandy matter 7 to 9 Putrescent matter (in partic- 4 to 8 ular waste vegetables and other vegetable matter) Plastics matter 4 to 6 Rubber 1 ______________________________________
Consequently, refuse contains, in every case, a significant percentage of combustible matter, which, statistically, would generally be on the order of 25% by weight.
The foregoing considerations have led to the idea of employing crushed refuse (previously rid of metallic components) as solid fuel, either as a powder, or in the form of briquettes for heating rotary cement kilns.
On the other hand, the ash obtained by the combustion of refuse contains mineral matter, especially calcium oxide, silica, alumina and ferrous oxide, found in clinkers. It has therefore been proposed to incorporate the ash from the refuse thus burnt, for heating the cement kilns into the starting ingredients for the manufacture of clinker. Methods founded upon these principles are described in the British Patents Nos. 1,450,294 and 1,510,392.
More precisely, British Patent No. 1,405,294 ('294), describes a method of manufacturing cement according to which the raw matter for the manufacture of clinker is introduced at one end of a rotary kiln and into the combustion zone of the kiln from the other end. A mixture of at least one fuel (e.g., pulverized coal, liquid hydrocarbon or fuel gas) intended for heating the kiln, a sufficient amount of air, and the crushed refuse is injected into the kiln. The mixture of refuse and fuel may reach 60% by weight fuel. The composition of the raw materials usually used in the manufacture of clinker is regulated in order to take into account the incorporation of ash resulting from the combustion of the crushed refuse.
In accordance with the method described in the British Patent No. 1,510,392 ('392), crushed refuse is employed as fuel for heating a cement kiln and the ash resulting from the combustion of the refuse is likewise incorporated into the raw materials for the manufacture of clinker. The difference between the method of '392 and the method described in the '294 is that the combustion of the refuse is carried out in a zone external to the cement kiln and not in the combustion zone of the kiln.
Thus, by the above methods, one is able to both solve the problem posed by the accumulation of refuse in public rubbish heaps and economically heating cement kilns.
In addition, it has been found that if the gases from combustion of the refuse are made to pass through the rotary cement kiln, a rate of emission of pollutant gas may be obtained which is less than that which is observed in the usual installations for pyrolizing refuse.
However, the proportion of refuse which one can eliminate in the above-mentioned manner with respect to the amount of clinker produced in the kiln remains limited to less than about 10%. Beyond such a percentage, the pollution released by the combustion of the refuse becomes too great and the composition of the clinker containing the ash from refuse departs from established standards.
It is therefore an object of the present invention to enable incorporation of larger amounts by weight of refuse into the starting ingredients for the manufacture of cement, without changing the composition of the clinker obtained, while practically completely eliminating the risk of pollution from the pyrolysis of the refuse in the firing furnace.